1. Field of the Invention
The present invention relates to optical communications, and more particularly to an optical coherent receiver for optical communications.
2. Description of the Related Art
With the gradual enhancement on the requirements of capacity and flexibility of the optical communication system, the coherent optical communication technology has become more and more important. In comparison with incoherent technology (such as on-off key, OOK) or auto coherent technology (such as differential quadrature phase-shift keying, DQPSK), the coherent technology has the following advantages: optical signal-to-noise ratio (OSNR) gain of approximately 3 dB; the capability to use more efficient modulation technologies (such as quadrature modulation, QAM) to enhance transmission capacity, and the capabilities to make convenient use of electric equalization technology in response to channel change, and lower production cost, etc. Like the case in electric coherent technology, an optical coherent receiver also requires a device to control the frequency of a local oscillator to let the difference (namely frequency offset) between this frequency and the frequency of a carrier wave be zero. However, in the optical communication system, there is no such information as the pilot in the wireless communication system to directly extract the frequency of the carrier wave, so that the first step in controlling the frequency offset in the optical coherent receiver is to estimate the frequency offset from the received signal. The characteristics of the optical communication system put the following demands on the frequency offset estimating apparatus in the optical coherent receiver. First, due to such problems of the laser as the temperature stability and aging, etc., the frequency offset might be as high as −5 GHz to +5 GHz in the actual system. Second, due to the non-stop transmission characteristics of the optical communication system, estimation of the frequency offset must be extremely precise and stable. Finally, due to the very high rate of the signal transmitted by optical communication, the corresponding AD rate and digital signal rate are also very high, so it is required that the computational complexity of the frequency offset estimating method has to be low. To sum it up, the optical coherent receiver requires a stable method and apparatus having a large range and a low computational complexity to perform frequency offset estimation.
FIG. 1 shows the position of a frequency offset estimating apparatus 110 in an optical coherent receiver. In the figure an optical frequency mixer 102, a local oscillator 103, photoelectric detectors 104, 105, analog-to-digital converters (ADC) 106, 107 and a controller 112 constitute a front end processing section (front end processor) 118 of the coherent receiver. The front end processing section 118 changes an optical signal 101 into a base band digital electric signal I+jQ 108, where I is a inphase component and Q is a quadrature component. The frequency offset estimating apparatus 110 estimates a numerical value 111 of the frequency offset in accordance with the base band digital electric signal I+jQ 108, and transmits it to the control module 112 to control the frequency of the local oscillator so that the frequency offset will be zero. A data recovery 109 restores data and outputs the restored data.
FIG. 2 illustrates a method for realizing a frequency offset estimating apparatus as proposed by Andreas Leven, et al., (“Frequency Estimation in Intradyne Reception”, IEEE Photonics Technology Letters, Volume: 19, No. 6, Mar. 15, 2007, pages 366-368). In FIG. 2, a register 201 and a conjugate calculator 202 delay an inputted base band electric signal by one symbol period, and obtain its conjugate to obtain a signal 207. The signal 207 is then multiplied at a multiplier 203 with an inputted base band electric signal to obtain a signal 208. The foregoing delaying, conjugating and multiplying calculations remove phase noise (the phase of the local oscillation and carrier wave randomly change, and can be considered as constant within several adjacent symbols). The phase of the signal 208 contains a difference between data information of two adjacent symbols and a phase offset within one symbol period introduced by the frequency offset. Subsequently, a quartic calculator 204 removes the data information, a complex summer 205 sums N data of a signal 209 to remove the influence of noise, and finally a phase change 111 within one symbol period and introduced by the frequency offset is obtained by a ¼ argument calculator 206. Since the symbol period (namely 1/symbol rate) is an invariant value to an optical transmission system, the phase change 111 directly represents the frequency offset to be estimated. For example, if the value of the phase change 111 is θ, the corresponding frequency offset will be (θ/2π)×Br (where Br indicates the symbol rate). There are two problems in the foregoing prior art method. First, since the output range of the ¼ argument calculator 206 is [−π/4, +π/4], the range of the frequency offset estimable by this method is [−Br/8, +Br/8]. The highest symbol rate achievable in the currently available optical transmission is 20 G symbol/second, and, taking such a system as an example, the range of the frequency offset estimable by this method is mere [−2.5 GHz, +2.5 GHz], which falls far short of the required [−5 GHz, 5 GHz]. Second, this method contains not only the multiplying calculation of complex numbers but also the quartic calculation of complex numbers, so that such computational complexity is far heavier than addition and subtraction calculation or logical calculation performed on real numbers. Insofar as the current digital signal processing technology is concerned, it is almost impossible to perform such complex calculations on symbols having rates as high as 10 G or 20 G symbol/second in the optical transmission system.